Fabrication of enriched 170Yb2O3 thin targets for studying asymmetric fission in sub-lead mass region

Owing to the highly oxidizing and chemically active nature of rare-earth elements, preparation of isotopically enriched thin targets of rare-earth metals has been a challenging task. The successful fabrication of thin films of enriched 170Yb2O3 on carbon backing with areal density in the range 160–200 μg/cm2 and approximately 22μg/cm2, respectively is described here along with the various challenges encountered during the fabrication and new optimized operational parameters. On account of limited availability and high cost of the enriched 170Yb due to its extremely low natural abundance, many iterations were attempted with natural ytterbium to optimize the evaporation setup and its associated parameters. The electron beam physical vapour deposition technique in ultra-high vacuum facility was found suitable for the evaporation and a specially designed cylindrical tantalum crucible was used to further improve the collection efficiency. Optimization trials were carried out with the deposition parameters reported in the literature but a considerably high concentration of tantalum was noticed in the films. This impurity could drastically contaminate the experimental data and needed to be minimized. The present work reports a new set of deposition parameters optimized for the fabrication of thin ytterbium oxide films with the tantalum contamination reduced to indiscernible amounts. The final evaporation of enriched 170Yb2O3 was carried out with a 30 mg of the material that was found to be fully consumed. Various characterization techniques were carried out to ascertain thinness, purity and the elemental composition of the films viz. Rutherford Backscattering Spectroscopy (RBS) and Energy Dispersive X-ray Spectroscopy (EDS). These measurements corroborated the achieved optimization of deposition parameters with negligible contribution of impurities in the films. The purity and stability of the films were further established with a good quality of data obtained after irradiation with 28Si beam.